The purpose of this project is to continue our studies of mammalian renal medullary function and to pursue two broad objectives. The first is the urinary concentrating mechanism. Investigations of its three principal elements are planned. The loop of Henle will be studied in Psammomys and Perognathus to determine the mechanism of concentration. The effect of antidiuretic hormone on the permeability and reflection coefficient of the collecting tubule to urea will be examined. Capillary blood flow in vasa recta under different conditions will be determined. Studies of the role of prostaglandins in each of the three elements are also planned. The second broad objective is to study medullary recycling of potassium and its role in urinary potassium excretion. All studies will be performed in vivo. Those of the tubule will employ micropuncture, microperfusion and related microanalytical techniques, including electron microprobe analysis. Investigations of vasa recta blood flow will also make use of television microscopy. In several studies, mathematical models will be formulated.